A low density parity check code (LDPC) was first introduced in the early 1960's by Gallanger. This code belongs to a linear block code, and its parity check matrix is formed of a very small number of 1. In the 1990's, e.g., Mackay proved that error performance of a low-density parity check code having very long length of code words was able to approach the Shannon's limit. Accordingly, the application of this code to next generation wireless communication systems has been expanded. For example, the code is employed for Europe digital video broadcasting-satellite (DVB-S) and WiMAX as one of the standards for the portable internet services.
The LDPC has a very large encoding complexity, and thus the LDPC is formed as a repeat accumulate (RA) code, thereby performing the encoding operation easily. As well-known through various documents, an error floor phenomenon is generated in the LDPC. Unlike turbo codes, a tripping set generated by a special structure of sum-product algorithm (SPA) and multiple effects of noise patterns is pointed out to be the main factor in the error floor phenomenon of the LDPC.
Accordingly, there has been a technical means for effectively overcoming the error floor phenomenon which causes performance deterioration of the LDPC, particularly systematic LDPC, of which the usage is increased.